The sharks stream in the thousands up South Florida’s coast, a sight that might terrify the people playing in the surf less than a football field away.

From a Cessna 172 flying slowly along the beach, Stephen Kajiura videotapes this procession of oceanic predators as they engage in their annual migration from North Carolina.

Kajiura, an associate professor of biology at Florida Atlantic University, is conducting the first systematic study of the migrations of blacktip sharks, a pattern that has led lifeguards to close beaches and may be linked to increases in shark bites.

“Every year we see the same thing, large numbers of sharks off South Florida,” he said. “We’ve got this really strong, seasonal influx of sharks. They spend the winter here and go up north again. We’ve known about the phenomenon for years, but no one’s ever studied it.”

The migrations run from roughly Cape Hatteras, N.C., to Broward County, or possibly northern Miami-Dade County, Kajiura said. The sharks start arriving in December, with numbers peaking in January and February before tapering off. By April, very few are left in South Florida. They’re following their prey, largely schools of mullet.

Since February of 2011, Kajiura or a friend has piloted the Cessna every two weeks on a route from Boca Raton to Jupiter Inlet with a video camera mounted on the plane. Back at the office, he analyzes the tapes, counting the sharks and noting their locations.

The numbers are shocking. On the strip of ocean he studies, running from shore to about two football fields out, he has counted 15,000 sharks on a single trip.

“That’s a huge number of sharks,” he said. “They are very close to shore. They’re sometimes right in the surf area. Sometimes 30 feet from shore….If you’re sitting in the water you have an average of one shark within 60 feet of you.”

The sharks are primarily blacktips, which reach a maximum length of about six feet, big enough to be dangerous.

They account for 20 percent of the unprovoked shark attacks in Florida, tying with bull sharks as the species responsible for the most attacks, according to the International Shark Attack File at the Florida Museum of Natural History. There were 11 unprovoked shark attacks in 2011 in Florida, the lowest number since 1993.

Blacktip attacks tend to be cases of mistaken identity, when murky water allows a shark to mistake a hand or foot for a fish. Such attacks typically take place off Central and Northern Florida, where the water is murkier and the possibility of errors easier. South Florida’s water is pretty clear, so the danger of an encounter with a confused blacktip is much less, Kajiura said.

“I don’t want to cause panic,” Kajiura said. “There’s a lot of sharks out there, but the probability of being bitten is very small.”

Blacktip attacks are rarely if ever fatal – although certainty is impossible because the species in shark attacks isn’t always known. The “big three” for killing people are the great white, tiger and bull shark, all of which can be found in Florida waters. These species tend not to travel in schools like blacktips, although they have come close enough to shore to attack swimmers.

The last fatal attack in South Florida took place in 2010, when an eight or nine foot bull or tiger shark killed a kiteboarder off Stuart.

Kajiura plans to submit his results for publication this year. Following that, if he can obtain the funding, he plans to fit sharks with transmitters so he can find out how far individual sharks travel and do a deeper study of their life histories.

Blacktip shark

Habitat: Found worldwide. Inhabit bays, lagoons, coastal waters

Food: fish, small sharks, rays, squid, crabs, octopus, lobster

Behavior: A fast swimmer, can be seen jumping out of the water.

Size: up to 6 feet

Commercial value: Caught for meat and fins.

Conservation status: Listed as “near threatened” by International Union for Conservation of Nature due to popularity as a food fish.